Studies of factors affecting pathogenicity and of pathogenicity mechanisms are being conducted by cytologic, immunologic, and biochemical methods. They include: (1) Attempts to restore infectivity in procyclic forms of Trypanosoma rhodesiense and T. gambiense by cultivating them with Tse-tse salivary glands. (2) Investigation of the host plasma components accreted on the surfaces of Try anosoma congolense and Trypanosoma brucei subspecies and of the significance of these components for the parasite and its interaction with the host. (3) Further studies of the common and unique antigens in the bloodstream and procyclic forms of T. brucei, T. rhodesiense, and T. congolense. (4) Chemical analysis of the antigenically active proteins released by living Leishmania, and investigations of the effects of these proteins on the immune responses of mice to Leishmania. (5) Analysis of the chemical composition of the surface of Trichomonas vaginalis by plant lectin agglutination and by cytochemical methods applicable at the fine-structural level.